Although variety of mass spectroscopic techniques exists, they address specific and limited analytical applications. Measurement of hydrogen stable isotopes (deuterium, 2H, and protium, 1H) has been problematic at low levels of 2H enrichment even by isotope ratio mass spectrometry (IRMS). The limitation of IRMS has been a large sample size requirement and a tedious and complicated purification/reduction scheme necessary to prepare samples as hydrogen gas (H2). This archaic methodology has severely limited the application of 2H-labeled materials as tracers of metabolic processes in the biomedical disciplines. Recent advances using continuous flow IRMS have greatly improved our ability to measure small enrichments of 13C and 15N isotopes in biological materials. These developments have been extended to couple a pyrolysis-reduction oven to an IRMS specially modified for measuring 2H. Availability of such a technique would greatly enhance and expand the research of biomedical investigators at the University of Vermont studying the regulation of metabolism in humans. An additional important feature of the new pyrolysis-IRMS is the rapid measurement of 2H enrichments in physiological water samples for determination of free-living energy expenditure using doubly (2H218O) water administered orally to humans. This technique allows evaluation on the effect of therapies upon energy expenditure in humans in their normal lives and makes it possible to test the efficacy of therapies in large groups of individuals that can be extrapolated to populations of patients. However, large numbers of subjects must be studied using doubly labeled water than our conventional IRMS system can handle. The new pyrolysis-IRMS would allow studies to be performed and measured at reasonable cost in a reasonable period of time in appropriately sized samples of patients. A pyrolysis-IRMS instrument is capable of measuring both low levels of 2H enrichment in both physiological-water samples and in metabolites of in vivo 2H tracer studies is requested for purchase to benefit a variety of investigators.